Low read current architecture for memory

ABSTRACT

A low read current architecture for memory. Bit lines of a cross point memory array are allowed to be charged by a selected word line until a minimum voltage differential between a memory state and a reference level is assured.

FIELD OF THE INVENTION

The present invention relates generally to sensing a signal in a memory array. More specifically, the present invention relates to sensing a signal in a two-terminal memory array.

BACKGROUND OF THE INVENTION

Data storage in high-density memory devices can be accomplished using a variety of techniques. Often, the technique used depends on whether or not the stored data is volatile or non-volatile. In volatile memory devices, such as SRAM and DRAM, for example, stored data is not retained when power is removed from the memory device. On the other hand, for non-volatile memory devices, such as MRAM and Flash devices, stored data is retained when power is removed from the memory device.

Resistive state memory devices are a promising new type of non-volatile memory in which data is stored in a memory element as a plurality of resistive states. A first resistive state can represent a logic “1” and a second resistive state can represent a logic “0”. The first and second resistive states can be set by applying a write voltage of a predetermined magnitude, polarity, and duration across the memory element during a write operation. For example, voltage pulses can be used to write a logic “1” and a logic “0”, respectively.

In either case, after data has been written to the memory element, reading the value of the stored data in the memory element is typically accomplished by applying a read voltage across the memory element and sensing a read current that flows through the memory element. For example, if a logic “0” represents a high resistance and a logic “1” represents a low resistance, then for a constant read voltage, a magnitude of the read current can be indicative of the resistive state of the memory element. Therefore, based on Ohm's law, the read current will be low if the data stored is a logic “0” (e.g., high resistance) or the read current will be high if the data stored is a logic “1” (e.g., low resistance). Consequently, the value of the stored data can be determined by sensing the magnitude of the read current.

In high density memory devices, it is desirable to pack many memory cells in a small area in order to increase memory density and data storage capacity. One factor that can have a significant impact on memory density is the number of terminals that are required to access a memory element for reading or writing. As the number of terminals required to access the memory element increases, device area increases with a concomitant decrease in areal density. Most memory technologies, such as DRAM, SRAM, and some MRAM devices, require at least three terminals to access the core memory element that stores the data. However, in some memory technologies, such as certain resistance based memories, two terminals can be used to both read and write the memory element.

An array of two terminal memory elements can include a plurality of row conductors and a plurality of column conductors and each memory element can have a terminal connected with one of row conductors and the other terminal connected with one of the column conductors. The typical arrangement is a two terminal cross-point memory array where each memory element is positioned approximately at an intersection of one of the row conductors with one of the column conductors. The terminals of the memory element connect with the row and column conductors above and below it. A single memory element can be written by applying the write voltage across the row and column conductors the memory element is connected with. Similarly, the memory element can be read by applying the read voltage across the row and column conductors the memory element is connected with. The read current can be sensed (e.g., measured) flowing through the row conductor or the column conductor.

One challenge that arises from a two-terminal configuration is that memory elements that share a row or column conductor with the memory element being read will also have a potential difference across their respective row and column conductors. The adjacent memory elements can be referred to as half-selected memory elements. The potential difference across the terminals of half-selected memory elements can cause half-select currents to flow through those memory elements. The half-select currents are additive and can be considered as a leakage current that occurs during a read operation. In a high density memory device, the number of memory elements in an array can be several thousand or more. During a read operation to a selected memory element in the array, the half-select currents from half-selected memory elements in the same row or same column as the selected memory element can vastly exceed the magnitude of the read current flowing through the selected memory element. The read current can be considered to be a signal and a magnitude of that signal is indicative of a data value of the data stored in the selected memory element. On the other hand, the leakage current can be considered to be noise that masks the read current signal. Therefore, in a large array, a signal-to-noise ratio (S/N) of the read current to the leakage current is low. A low S/N ratio can make it difficult to distinguish between the read current and the leakage current. Consequently, the low S/N ratio makes it difficult to detect an accurate value for the stored data.

There are continuing efforts to improve accuracy in reading data and in increasing S/N ratios in memory arrays having leakage current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of an integrated circuit 100 that performs memory functions.

FIG. 1B is a block diagram of the memory array circuitry depicted in FIG. 1A during a single-cell read.

FIG. 2 is a block diagram of a cross point array during a single-cell write.

FIG. 3A is a block diagram of an exemplary cross point array configuration that includes memory elements electrically coupled with a sense amplifier in the peripheral circuitry.

FIG. 3B depicts an exemplary memory array reduced to a simple RC (resistor/capacitor) network.

FIG. 3C depicts a voltage-time graph of the response of the RC network depicted in FIG. 3B.

FIG. 4 is a block diagram depicting an exemplary high-latency cross point array.

FIG. 5 depicts an current-voltage (I-V) graph of an exemplary memory element, with which may be used to populate the cross point array.

FIG. 6A depicts a voltage-time graph of an exemplary reference voltage in relation to an exemplary “1” cell voltage band and an exemplary “0” cell voltage band.

FIG. 6B depicts a voltage-time graph of the margin between the exemplary reference voltage and the worst-case scenarios of the “1” cell voltage band and the “0” cell voltage band.

Although the previous drawings depict various examples of the invention, the invention is not limited by the depicted examples. The depictions are not necessarily to scale. Like elements are identified with like reference numerals

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are set forth to provide a through understanding of the present invention. It will be apparent to one skilled in the art that the present invention may be practiced without some or all of these specific details. In other instances, well known elements and process steps have not been described in depth in order to avoid unnecessarily obscuring the present invention.

FIG. 1A is a block diagram of an integrated circuit 100 that performs memory functions. Memory array circuitry 105, 110, 115, and 120, which are typically fabricated on a semiconductor substrate 125, are components of the integrated circuit 100. Memory array circuitry 105-120 includes arrays of memory cells and associated peripheral circuitry. A bus 130 connects the memory array circuitry 105-120 to input pads 132 and output pads 134. Input pads 132 and output pads 134 are connected to input pins 135 and output pins 140 through interconnects 144, such as wire bonding. Input and output pins 135 and 140 are typically part of a lead frame 142 that enables the semiconductor substrate 125 to adhere to a form factor requirement of a system.

An input signal from a system that wants to access the memory on the integrated circuit 100 enters the lead frame 142 through input pins 135, where the signal is electrically communicated by interconnects 144 to the input pads 132. The bus 130 carries the input signal to the memory array circuitry 105, 110, 115, and 120. The memory array circuitry 105-120 produces an output signal, which is carried by bus 130 to output pads 134. The output signal is then electrically communicated by interconnects 144 to output pins 140.

FIG. 1B is a block diagram of the memory array circuitry 105 depicted in FIG. 1A during a single-cell read. The memory array circuitry 105 includes a cross point array 145, which has a number of x-direction array lines 150, a number of y-direction array lines 155, a number of memory cells 175, an x-direction decoder 160, a y-direction decoder 165, and sensing circuitry 170. In a typical cross point array 145, the x-direction array lines 150 and the y-direction array lines 155 are preferably oriented substantially perpendicular to each other. Moreover, the x-direction array lines 150 can be positioned above or below the y-direction array lines 155. The array lines need neither be equal in number nor be proportional in lengths to that of the present depiction.

The cross point array 145 includes a number of memory cells 175, each containing a memory element 172. In a typical cross point array, each memory element 172 is positioned at an intersection of a single x-direction array line 150 with a single y-direction array line 155 and is electrically coupled with its respective array lines. Each memory element 172 is selected for a data operation (e.g., a read or write access) by applying a signal to its respective array lines. The cross point array 145 is a type of memory array that is generally suitable for memory cells 175 that can be accessed with only two terminals. Memory arrays that require more than two terminals require more area for the routing of additional array lines and typically have different layout requirements that cannot be met with cross point arrays. Cross point array 145 can be a single layer of memory cells or a stacked cross point array that includes multiple layers of memory cells that are stacked upon each other. For example, the multiple layers can be vertically stacked along a z-axis. Memory cells in a stacked cross point array can either be electrically isolated from its vertically adjacent layers or can share array lines.

The x-direction decoder 160 and the y-direction decoder 165 transform control signals into signals usable by the cross point array 145. The sensing circuitry 170, usually designed to perform signal amplification, is electrically coupled with the y-direction array lines 155 and is operative to output signals to the bus 130.

Input signal(s) entering the memory array circuitry 105 typically first go through the x-direction decoder 160 and the y-direction decoder 165 via bus 130. The decoded signal(s) then enter cross point array 145, where the signal(s) access appropriate memory cell(s) 175 through the x-direction array line(s) 150 and the y-direction array line(s) 155. Sensing circuitry 170 detects at least one signal and generates an output signal to the output pads 134 via bus 130.

In FIG. 1B, a single memory cell 175 is selected using a selected x-direction array line 185 and a selected y-direction array line 180 (both depicted in heavy line). The selected memory cell 175 includes a selected memory element 172. In the depicted embodiment, the x-direction array line 185 is energized to approximately −2 V by a voltage source and the y-direction array line 180 is energized to approximately +2 V by another voltage source. A current flows through the selected memory element 172. The sensing circuitry 170 detects the current, performs the appropriate amplification of the current, and generates the output signal that is indicative of a binary value of “0” or “1”. Although not shown, a page mode access can be accomplished by energizing all y-direction array lines 155 at approximately +2 V and simultaneously sensing the signals on each y-direction array line 155. The actual magnitudes of the voltages used on the selected x-direction 185 and y-direction 180 array lines will depend upon the specifications of cross point array 145 and memory element 172. Similarly, different architecture will use different or alternating polarities.

FIG. 2 is a block diagram of a cross point array 145 during a single-cell write. Each selected memory cell 230 includes a selected memory element 235, which is located at the intersection of a selected x-direction array line 205 and a selected y-direction array line 215. The remaining x-direction array lines 210 and the remaining y-direction array lines 220 are unselected. For example, the array lines 210 and 220 can be electrically coupled to a ground potential. Ground is defined as any baseline reference voltage and is commonly 0 V. For clarity, FIG. 2 does not show unselected memory elements located at the intersections of the unselected x-direction array lines 210 and the unselected y-direction array lines 220.

In the depicted embodiment, a +3 V (or −3V) voltage source energizes the selected x-direction array line 205 and a −3 V (or +3V) voltage source energizes the selected y-direction array line 215. Consequently, the memory cell 230 is a selected memory cell (and the memory element 235 is a selected memory element). A voltage source coupled with the selected x-direction array line 205 can apply a voltage potential of +3V (or −3V) and the voltage source coupled with the selected y-direction array line 215 can apply a voltage potential of −3V (or +3V). The resulting potential difference across the selected memory element 235 can be, depending upon the specifications of the memory element, sufficient to trigger the memory element 235 to switch states, thereby performing a write operation. It should be noted that a leakage current can flow through unselected memory elements.

Additionally, the data value written to the selected memory element 235 can depend on the magnitude and polarity of the voltages applied to the array lines 205 and 215. Using the example above, if the voltage applied to the selected x-direction array line 205 is +3 V and the voltage applied to the selected y-direction array line 215 is −3 V, then, for some memory elements, a binary “1” would be written to the selected memory element 235. On the other hand, if the voltage applied to the selected x-directional array line 205 is −3 V and the voltage applied to the selected y-directional array line 215 is +3 V, then a binary “0” would be written to the selected memory element 235. Memory elements using different materials, having different properties and/or being arranged differently could require higher or lower voltages and/or opposite polarities. For some embodiments, modifying magnitude and duration of the applied voltage will also produce intermediate values such as a binary value of “00,” “01,” “10” or “11. The process, although not shown, can be designed to simultaneously write to multiple memory elements. One method, the “two-cycle write,” writes all the “1”s in one cycle and all the “0”s in another cycle.

Further description of one possible memory element can be found in U.S. patent application Ser. No. 11/095,026, filed Mar. 30, 2005, and titled “Memory Using Mixed Valence Conductive Oxides,” hereby incorporated by reference in its entirety and for all purposes. The application describes non-volatile third dimension memory cells that can be arranged in a cross-point array. The application explains that a two terminal memory element can change conductivity when exposed to an appropriate voltage drop across the two terminals. The memory element includes both a mixed ionic electronic conductor and a layer of material that has the bulk properties of an electrolytic tunnel barrier (properties of an electronic insulator and an ionic conductor). A voltage drop across the electrolytic tunnel barrier causes an electrical field within the mixed ionic electronic conductor that is strong enough to move oxygen ions out of the mixed ionic electronic conductor and into the electrolytic tunnel barrier. Movement of oxygen causes the memory element to change its conductivity.

FIG. 3A is a block diagram of an exemplary cross point array configuration that includes memory elements electrically coupled with a sense amplifier 350 in the peripheral circuitry. In this depiction, a cross point array 300 has one selected x-direction array line 305 (depicted in heavy line), 1023 unselected x-direction array lines 310, a y-direction array line 320 (depicted in heavy line) that is held at approximately 0 V, and memory cells 330. Each memory cell 330 includes a memory element 335, which is placed at the intersections of one of the x-direction array lines (305, 310) and the y-direction array line 320. Although not depicted, the array 300 may include additional y-direction array lines which might or might not be sensed simultaneously.

A reference voltage V_(REF) is electrically coupled with one of the inputs of the sense amplifier 350 and the voltage applied to the y-direction array line 320 is electrically coupled with another input of the sense amplifier 350. The memory element 330 may include a non-ohmic device, as described in “High Density NVRAM,” U.S. application Ser. No. 10/360,005, filed Feb. 7, 2003, now U.S. Pat. No. 6,917,539, incorporated herein by reference in its entirety and for all purposes. The non-ohmic device exhibits a very high resistance for a certain range of voltages and a very low resistance for voltages outside that range. A variable resistance of the memory element 335 on the selected x-direction array line 305 is denoted as R. Current will flow from the selected x-direction array line 305 to the y-direction array line 320, which is initially charged to the unselected word line potential (“precharged”), which is about 0 V. In a preferred embodiment, the y-direction array line 320 floats from ground to a voltage much less than the read voltage on the selected x-direction array line 305 (depicted as 1 V in FIG. 3A), but greater than 0 V. In such an embodiment, a small amount of current will flow from the y-direction array line 320 to the unselected x-direction array lines 310.

FIG. 3B depicts an exemplary memory array 300 reduced to a simple RC (resistor/capacitor) network. In order for the sense amplifier 350 to produce a useful output with an acceptable access time, prior art techniques would require a difference of at least 50 mV between a reference voltage (V_(REF)) and the y-direction array line 320 voltage (V_(BIT LINE)). Assuming R for a “1” is about 100 kΩ and R for a “0” is about 1 MΩ, then the unselected memory elements can be represented as a capacitor (for the intrinsic capacitance) and a single resistor having the value of R/1023 (since there are 1023 unselected lines in FIG. 3A). The combined value of R/1023 will have a maximum value when all of the unselected lines are “0” and a minimum value when all of the unselected lines are “1”. Y-direction array line 320 will have an associated capacitance, which can have an exemplary value of 1 pF. If the exemplary resistance is 100 kΩ for a “1” state, the initial current charging the associated capacitance is 10 μA (1 V/100 kΩ). If the exemplary resistance is assumed to be 1 MΩ for a “0” state, the initial current charging the associated capacitance is 1 μA (1 V/1 MΩ). The rate of change of y-direction array line 320 voltage is determined by current divided by capacitance (I/C). To reach 100 mV the time required for a memory cell in the “1” state would be 10 μs, which can be derived from [(100 mV)×(1 pf)/10 μA]. Since the rate of change of y-direction array line 320 voltage is higher with higher current, rate of change can be used to determine the memory cell's state. This is depicted in FIG. 3C, which is a voltage-time graph of the response of the RC network depicted in FIG. 3B.

FIG. 4 is a block diagram depicting an exemplary high-latency cross point array 400. The cross point array 400 includes a selected x-direction array line 410 (depicted in heavy line), a number of unselected x-direction array lines 420, a number of y-direction array lines 415 (depicted in heavy line), and a number of memory cells 430. Each memory cell 430 includes a memory element 450, which is positioned at the intersection of one of the x-direction array lines (410, 420) and one of the y-direction array lines 415.

In this depiction, the selected x-direction array line 410 is energized by a voltage source at 2 V while the unselected x-direction array lines 420 are grounded at approximately 0 V. The magnitude and polarity of the voltage potentials applied to the array lines (410, 415, and 420) will be application and material dependent. The y-direction array lines 415 are precharged to the same voltage as the unselected x-direction array lines 420 (approximately 0 V). A current I_(SEL) flows through each selected memory element 450 that is positioned at the intersection of the selected x-direction array line 410 and the y-direction array lines 415. As the y-direction array lines 415 are charged by the selected x-direction array line 410, another current I_(UNSEL) flows through the unselected memory elements 450 positioned at the intersection of one of the unselected x-direction array lines 420 and one of the y-direction array lines 415. In the configuration depicted, I_(SEL) is typically much higher than I_(UNSEL). In a preferred embodiment, all the I_(UNSEL) together (Σ I_(UNSEL)) flowing from a single y-direction array line 415 is less than I_(SEL) from that y-direction array line 415. Since R (see FIG. 3A) can be representative of a “0” or a “1”, I_(SEL) flowing through each memory element 450 along the selected x-direction array line 410 need not be equal in magnitude. Likewise, I_(UNSEL) flowing through each memory element 450 along the unselected x-direction array lines 420 need not be equal in magnitude.

Multiple y-direction array lines 415 can be read simultaneously to attain a faster read rate. In such a multi-sensing read, the total current on the selected x-direction line 410 is the sum of the currents on all of the memory elements 450 located along the selected x-direction line 410. Generally, current density of array lines increases with narrower width of array lines. However, technological issues such as electromigration can become a significant hindrance to the reliability of the integrated circuit if current density exceeds a desirable limit. Because narrow width of array lines is advantageous in increasing the areal density of the array 400, it is preferable to read the memory elements 450 at a low total current in order to keep current density low. Furthermore, arrays with lower currents have lower power consumption and lower heat dissipation.

FIG. 5 depicts a current-voltage (I-V) graph of an exemplary memory element 450, with which may be used to populate the cross point array 400. As can be seen on the graph, the memory element 450 produces a non-linear I-V curve 510 whereas an ideally resistive device produces a linear I-V curve 520. Accordingly, the resistance of memory element 450 is a non-linear function of the voltage applied across the memory element 450. It is possible to achieve a much lower current at low voltage inputs by using a memory element 450 that has a non-linear resistance as a function of applied voltage rather than by using a linear resistive element.

As shown in FIG. 4, a voltage of 2 V generates a current I_(SEL) through the memory elements on the selected x-direction array line 410, and the 100 mV applied to the y-direction array lines 415 generates a current I_(UNSEL) through memory elements 450 on the unselected x-direction array lines 420. The ratio of I_(SEL) to I_(UNSEL) can reach approximately 1,000,000 for the configuration depicted in FIG. 4. It is generally desirable to obtain as high a ratio as possible for I_(SEL) to I_(UNSEL). Because of the nonlinear nature of the memory element, an I_(SEL) to I_(UNSEL) ratio can be substantially higher than an I_(SEL) to IHALF-SEL ratio.

FIG. 6A depicts a voltage-time graph of an exemplary reference voltage (V_(REF)) 630 in relation to an exemplary “1” cell voltage band 610 and an exemplary “0” cell voltage band 620. The voltage detected by each sense amplifier 460 (see FIG. 4) is a result of all the voltage drops along its y-direction array line 415. If the memory cell 430 is in the “1” state, then the sense amplifier 460 could receive a voltage anywhere within the “1” cell voltage band 610. Precisely where the voltage will fall within the band will depend on the individual states of the unselected memory cells. Similarly, if the memory cell 430 is in the “0” state, then the sense amplifier could receive a voltage anywhere within the “0” cell voltage band 620, depending on the states of the unselected memory cells. The shape of each band will, of course, depend on the specific features of the memory cells that are used. The depicted reference voltage 630 assumes a column of reference cells programmed to a mid-point between a “0” state and a “1” state, as described in “Two Terminal Memory Array Having Reference Cells,” U.S. application Ser. No. 10/895,218, filed Jul. 11, 2006, now U.S. Pat. No. 7,075,817, incorporated herein by reference in its entirety and for all purposes.

FIG. 6B depicts a voltage-time graph of the margin between the exemplary reference voltage 630 and the worst-case scenarios of the “1” cell voltage band 610 and the “0” cell voltage band 620. The worst-case margin between “0” cell and reference voltage 640 is the profile derived from a selected cell in the “0” state and 1023 unselected cells in the “1” state subtracted from a column of 1024 cells in a “reference” state (550 kΩ). Similarly, the worst-case margin between “1” cell and reference voltage 650 is the profile derived from a selected cell in the “1” state and 1023 unselected cells in the “0” state subtracted from a column of 1024 cells in the “reference” state. If the sense amplifiers 460 (see FIG. 4) require a minimum voltage differential of 50 mV, then a comfortable sensing window 660 would be between 40 μs and 340 μs, limited by the “1” cell margin 650. If desired, those skilled in the art can optimize the reference voltage 470 such that a different sensing window 660 is derived. A sensing window (the time period where a minimum voltage differential between a memory state and a reference level is assured) depends upon design choices that include the type of memory cell that is being used, the type of sense amp being used, what reference level is used, etc.

Referring back to FIG. 4, the selected x-direction array line 410 (which can be considered a word line) is energized to an appropriate x-direction read voltage, the unselected x-direction array lines 420 are brought to ground and the y-direction array lines 415 (which can be considered a bit line), initially at ground, is allowed to float for some time within a sensing window. The sense amplifiers 460 then output a signal to a register 480. Depending on width of the bus, a multiplexer 490 may then be used to ensure the output has the correct size. For example, if the output width is 8 bits, and if there are 1024 y-direction array lines 415, then the multiplexer 490 would output 128-bytes from the register 480. It should be appreciated that while the latency of the system would be relatively high due to the sensing window, once the register 480 was full, the throughput from the register to the bus would be very fast, limited only by the width of the bus and the cycle speed.

Although the invention has been described in its presently contemplated best mode, it is clear that it is susceptible to numerous modifications, modes of operation and embodiments, all within the ability and skill of those familiar with the art and without exercise of further inventive activity. Accordingly, that which is intended to be protected by Letters Patent is set forth in the claims and includes all variations and modifications that fall within the spirit and scope of the claim 

1. A method for reading data, comprising: precharging bit lines to a first voltage; energizing a selected word line to a second voltage different from the first voltage; charging unselected word lines to the first voltage; and allowing bit lines to be charged by current flow from the selected word line after the selected word line is energize.
 2. The method of claim 1, further comprising sensing the bit lines at a time when the period of time is within a sensing window.
 3. The method of claim 1, wherein the bit lines and word lines are part of a two terminal cross point array.
 4. The method of claim 1, wherein the first voltage is ground.
 5. A device for reading data, comprising: a two terminal cross point memory array comprising a plurality of bit lines and a plurality of word lines; a word line decoder electrically coupled to the plurality of word lines that is operable to energize a selected word line to a voltage and ground unselected word lines, wherein each of the plurality of bit lines has a select current associated with the select line and a total unselect current associated with the unselected word lines; and a sense unit electrically coupled to the plurality of bit lines and operable to sense data at a point in time that is within a sensing window; wherein the total unselect current is less than the select current at the point in time. 